Consequences of a Deficit in Vitamin B6 Biosynthesis de Novo for Hormone Homeostasis and Root Development in Arabidopsis  

Abstract

Vitamin B6 (pyridoxal 5′-phosphate) is an essential cofactor of many metabolic enzymes. Plants biosynthesize the vitamin de novo employing two enzymes, pyridoxine synthase1 (PDX1) and PDX2. In Arabidopsis (Arabidopsis thaliana), there are two catalytically active paralogs of PDX1 (PDX1.1 and PDX1.3) producing the vitamin at comparable rates. Since single mutants are viable but the pdx1.1 pdx1.3 double mutant is lethal, the corresponding enzymes seem redundant. However, the single mutants exhibit substantial phenotypic differences, particularly at the level of root development, with pdx1.3 being more impaired than pdx1.1. Here, we investigate the differential regulation of PDX1.1 and PDX1.3 by identifying factors involved in their disparate phenotypes. Swapped-promoter experiments clarify the presence of distinct regulatory elements in the upstream regions of both genes. Exogenous sucrose (Suc) triggers impaired ethylene production in both mutants but is more severe in pdx1.3 than in pdx1.1. Interestingly, Suc specifically represses PDX1.1 expression, accounting for the stronger vitamin B6 deficit in pdx1.3 compared with pdx1.1. Surprisingly, Suc enhances auxin levels in pdx1.1, whereas the levels are diminished in pdx1.3. In the case of pdx1.3, the previously reported reduced meristem activity combined with the impaired ethylene and auxin levels manifest the specific root developmental defects. Moreover, it is the deficit in ethylene production and/or signaling that triggers this outcome. On the other hand, we hypothesize that it is the increased auxin content of pdx1.1 that is responsible for the root developmental defects observed therein. We conclude that PDX1.1 and PDX1.3 play partially nonredundant roles and are differentially regulated as manifested in disparate root growth impairment morphologies.

Balancing of B6 Vitamers Is Essential for Plant Development and Metabolism in Arabidopsis

“…A complete loss of vitamin B 6 biosynthesis de novo is embryo lethal (TambascoStudart et al, 2005;Titiz et al, 2006), while mutants in either one of the two catalytic PDX1 genes (PDX1.1 or PDX1.3) have short roots and are stress hypersensitive (Chen and Xiong, 2005;Titiz et al, 2006;Wagner et al, 2006;Boycheva et al, 2015). The latter phenotypes are remarkably similar to mutants in the salvage pathway kinase SOS4 (Rueschhoff et al, 2013), originally isolated due to their salt sensitivity .…”

“…The latter phenotypes are remarkably similar to mutants in the salvage pathway kinase SOS4 (Rueschhoff et al, 2013), originally isolated due to their salt sensitivity . Recently, the phenotypes associated with pdx1.1 and pdx1.3 have been linked primarily to a perturbation in ethylene and auxin homeostasis (Boycheva et al, 2015), two phytohormones that require PLP as a cofactor for their biosynthesis. By contrast, mutants partially impaired in the expression of the salvage pathway enzyme PDX3 were reported to be morphologically indistinguishable from wild-type plants when grown on soil under normal conditions but hypersensitive to high sucrose levels, high light, and chilling (González et al, 2007).…”

Long-Distance Transport of Thiamine (Vitamin B1) Is Concomitant with That of Polyamines

“…Histochemical GUS expression analyses was carried out essentially as described by Boycheva et al (2015). Images were captured with a Leica MZ16 stereomicroscope equipped with an Infinity 2 digital camera (Leica Microsystems).…”

Producing the Ethylene Signal: Regulation and Diversification of Ethylene Biosynthetic Enzymes

“…New inhibitors that may show improved specificity for ACS were recently identified (Lin et al, 2010); these compounds do not compete with PLP, reducing side effects on other PLP-dependent enzymes. Nonetheless, severe effects on seedling root growth should be expected for all of these inhibitors of ethylene synthesis, because reduced ethylene synthesis appears to be a primary defect in PLP-deficient roots, leading to impairments in auxin synthesis and distribution and in apical meristem function (Boycheva et al, 2015). Intriguingly, genetic analysis revealed an essential embryonic requirement for ACS function (but not ethylene signaling) in Arabidopsis (Arabidopsis thaliana; Tsuchisaka et al, 2009), and additional work has suggested that ACC may act in an ethylene-independent mode to signal loss of cell wall integrity (Xu et al, 2008;Tsang et al, 2011).…”